1. Introduction

Certain porous materials have special properties and functions that cannot normally be obtained by conventional dense counterparts. Therefore, porous materials are now used in many applications such as final products and in various technological processes. Macroporous materials are used in various forms and compositions in everyday life; e.g. polymeric foams, packaging, lightweight aluminum structures in buildings, aircraft, and as a porous ceramic for water [1,2].

A growing number of applications that require advanced ceramics have appeared in recent decades, especially in environments where high temperatures, extended wear and corrosive environments are present. Such applications include the filtration of molten metals, high temperature insulation, support for catalytic reactions [3], filtration of particulates from exhaust gases of diesel engines and filtration of hot gases in various corrosive industrial processes, for example [4-6]. The advantages of using porous ceramic for these applications are generally a high melting point, suitable electronic properties, good corrosion resistance and wear resistance in combination with the characteristics acquired by the replacement of the solid material by voids in the component. Such characteristics include low thermal mass, low thermal conductivity, permeability control, high surface area, low density, high specific strength and a low dielectric constant [1,7]. These properties can be tailored for each specific application by controlling the composition and microstructure of the porous ceramic [8,9].

According to the type of application different microstructures and thus different preparation methods are required for porous ceramic bodies. For thermal or acoustic insulators materials with closed porosity are preferred, whereas membranes and filters require open pores with exactly defined pore size, for example. Are currently used some specific techniques to obtain pores with polymer and cotton for use in thermal insulation [10,11]. On the other hand, for application to environmental issues sodium hydroxide is used to pore formation [12].

This work uses a simple and versatile methodology to prepare sintered porous ceramic bodies obtained by replica method using organic fibers. The porous bodies of Al2O3 and Al2O3-ZrO2 were synthesized through co-precipitation method [11] and replica [1,11] aiming to achieve a porous ceramic structure with thermal properties in agreement to the application of refractory materials.

2. Methodology

The Al2O3 fibers were prepared from an aluminum hydroxide slurry in which the organic fibers were dispersed and impregnated. The solvents were eliminated from the organic fibers through heat treatment and the remaining ceramic precursor was sintered to Al2O3.

The synthesis of aluminum hydroxide consisted of dissolving aluminum nitrate – Al(NO3)3.9H2O (Synth) - in water; this solution was heated in a heating plate at 80°C. After its complete dilution ammonium hydroxide - NH4OH (Vetec) - was added until pH = 9 was reached. The Al(NO3)-NH4OH ratio was 1:6 (in moles). Afterwards, anhydrous citric acid - C6H8O7 (Synth) - was added until pH = 1 was reached, keeping a Al(NO3)-C6H8O7 molar ratio of 3:1. The addition of the citric acid causes the precipitation of aluminum nitrate in aluminum hydroxide - Al(OH)3 - forming a slurry solution that was used to impregnate the organic fibers.

The organic fibers were impregnated in the aluminum hydroxide solution following the procedure of the replica method [1,11]. The organic fibers consisted of commercial cotton produced by Johnson & Johnson. After complete impregnation, the cotton fibers were pressed into a ceramic crucible to eliminate the surplus solution from the fibers and a green body with the shape of the recipient is obtained. These green bodies were placed on an electric oven (INTI, model FE-1300) for the calcination process, which occurred at 1200°C in air for 2 hours using a heating rate of 2°C/min. During this process there was a complete removal of the organic matter and α-alumina was formed from phase transition of aluminum hydroxide [13].

For the synthesis of zirconium hydroxide, initially, zirconium tetrachloride - ZrCl4 (Synth) - was diluted in water. Next, this solution was heated in a heating plate at 80°C and after complete dilution ammonium hydroxide - NH4OH-NH4OH (Vetec) - was added until pH = 9 was reached keeping a ZrCl4-NH4OH molar ratio of 1:6. Afterwards, anhydrous citric acid - C6H8O7 (Synth) - was added until pH = 1 was reached, keeping a ZrCl4-C6H8O7 molar ratio of 3:1. The addition of the citric acid causes the precipitation of zirconium tetrachloride in zirconium hydroxide - Zr(OH)4 - forming a slurry solution that was used to impregnate the Al2O3 fibers.

The calcinated Al2O3 fibers were impregnated with the zirconium hydroxide slurry to form a structure of Al2O3 covered with ZrO2. After impregnation, samples were sintered on the electric oven at 1200-1600°C in air for 4 hours using a heating rate of 2°C/min causing the transition of zirconium hydroxide to zirconia while the fibers are sintered [14].

2.1. Characterization

The crystalline phase was determined by X-ray diffraction (XRD) using a Shimadzu XRD-6000 diffractometer with CuKα; radiation at 40 kV and 40 mA with the patterns recorded in the 20 to 80 theta measuring range at a scan rate of 2°/min, at room temperature. Through micro-Raman was possible to verify the peaks corresponding to the symmetry of the ceramic phases; the analysis was performed in room temperature, using an wavelength of 514.5nm of an argon laser as exciting source. The energy was maintained in 15mW and a 50x lense was used. The spectra was registered through a monochromer T-64 Jobin-Yvon jointed to a CCD detector.

The morfology and structure of the pores created by the template of the metal oxides were analysed by FEG-SEM (Supra 35-VP, Carl Zeiss). Mean grain size was determined by using the intercept method.

A TG analysis of the porous alumina body was performed in a NETZSCH TG 209 F1 thermal analyzer using 10 mg samples heated to 25-900°C in air at a rate of 10°C/min.

Thermophysical properties are determined by the flash laser method [15-17] that allows to determine thermal diffusivity and specific heat of the sample. Specific heat and thermal diffusivity values were found through measurement of the increase in temperature of the opposite face of the material in the shape of a small disk, while the frontal face receives a strong energy flash by a laser. A laser with maximum power of 90 watts was applied. The irradiation time is within the order of 10 ms. The measurements were taken at room temperature in normal atmosphere.

The thermal diffusivity is calculated from the thickness of the sample and the required time for the temperature in the opposite face to reach 50% of the temperature in the laser incident face. Specific heat is determined by the density and thickness of the sample given the maximum temperature reached in the opposite face and the amount of heat received. Thermal conductivity is calculate by the product of the thermal diffusivity, specific heat and density, as shown in the equation 1 where K = thermal conductivity, α; = thermal diffusivity, ρ; = density and Cp = specific heat.

3. Results

Figure 1 shows the decomposition of organic fibers and aluminum hydroxide [18]. XRD patterns of samples in various temperatures suggest that phase changes occur at 200°C, 300°C, 400°C, 600°C, 700°C, 1100°C and 1600°C. The material phases agree with those phases obtained using the method of a solution precursor cation embedded in fibrous cotton organic matrix under high temperatures which profit from the reactive activity of decomposition and the reaction of fibers / aluminum hydroxide appears at 600°C but disappears at 700°C. At 700°C, the positions of all peaks agree with the positions of number 46-1212 of the JPCDS file (see Figure 1) which suggests that a completely crystallized Al2O3 product was obtained. A crystalline Al2O3 is obtained by the step-wise transition of Al(OH)3 + cotton fibers between 300°C and 700°C. The results indicate that Al2O3 is the rhombohedral space group R-3C (167). Peaks were observed relative to α-Al2O3, and its crystalline peaks are identified in the X-ray diffractogram shown in Figure 1 as determined by the Scherrer equation [19], the average crystallite size is ~ 350 nm at 1600°C.

Figure 1.

XRD of the sintered samples in different temperatures suggests the formation of alumina and zirconia phases in every temperature of sintering. The peaks for Al2O3-ZrO2 are in agreement with those found in PDF (Powder Diffraction File) 53-559 file [20]; α-Al2O3 agrees with what is found in PDF 46-1212 file [21]; ZrO2 agrees with the peaks in JPCDS 37-1484 file [22] (see Figure 2).

Figure 2.

X-ray diffraction patterns of the Al2O3-ZrO2 porous ceramics at several sintering temperatures.

Raman is a powerful technique to detect the allotropic forms of zirconia [23]. According to the previous work of Popa et al [24], densified regions are caused by presence of monoclinic zirconia. Figure 3 shows results regarding the measurements of micro-Raman spectroscopy dealing with wavelength radiation and vibration energies of the molecules. According to Raman results, vibration of Al-O bonds are related to the peaks described below: shows a peak at 378 cm-1, that may be considered polycrystalline α-Al2O3 [25]. For samples sintered above 1200°C there were significant spectroscopic bands in 410, 470, 605 and 630 cm-1, which are identified as absorption bands characteristic of α-Al2O3 [26-28]. These results are agreement with the study conducted by Cava et al, in which spectroscopic bands of α-Al2O3 are present in temperatures above 1000°C [13].

Figure 4 shows the morphological profile of the Al2O3 and Al2O3-ZrO2 porous body with an increase of 5000x where the samples are not similar. Densification and grain growth are two inversely proportional properties during the sintering process. The high porosity and low densification shown in SEM images of the Al2O3 porous bodies (Figure 4a) relates to a heating rate of 2°C/min which was used during the sintering process [29]. SEM micrographs (Figure 4b) show the presence of densified regions of zirconia; therefore, micro-Raman was used to identify the presence of monoclinic zirconia in these regions. Vibration of Zr-O bonds are related to the peaks on the spectroscopic bands of monoclinic zirconia were identified in frequencies of 505, 534, 550 cm-1 [24,30], peaks shown in temperatures of 1300-1600°C. Only a small amount of monoclinic zirconia is present; however it can be suggested that this amount was enough to densify the regions of porous ceramics agglomerating around grains [24]. Tetragonal zirconia was clearly detected in the peaks 260, 300, 323 and 340 cm-1 [24] increasing their intensity above 1300°C. It was found only two well defined peaks at 217 and 745 corresponding to cristobalite zirconia between temperatures of 1200-1600°C. The crystalline phase of Al2O3-ZrO2 shown in Figure 2 has a cubic structure [20], not being detected by Raman.

Figure 4.

SEM micrographs of Al2O3 and Al2O3-ZrO2 porous bodies at temperature of 1600°C at increasing of 5000x.

The method of embedding a cotton fiber in the organic matrix is based on the impregnation of a cellular structure with a ceramic suspension or solution precursor ceramic to produce a macroporous ceramic which has the same morphology as the original porous material (cotton) as illustrated in SEM images of Figure 4. Many cellular structures can be used as templates to produce macroporous ceramic embedding techniques for organic matrices.

The process of manufacturing ceramic fibers using cotton as a template by employing the embedding of the organic matrix [1] can be divided into two stages: the formation of the Al2O3 fiber morphology and the removal of the cotton used as a template.

The TG illustrated in Figure 5 shows the weight loss along with images of samples at temperatures ccurring in the formation of pore bodies at a temperature of 900°C; a gradual weight loss of weight is evident during the process. The sample weight loss was 82.5% up to 650°C where it terminated the decomposition of the carbonaceous mass with no significant loss of mass after the complete elimination of the carbonaceous mass.

Figure 5.

Thermogravimetric analysis of the Al2O3 porous bodies.

Al2O3 fibers are formed during the calcination process. After the formation of Al2O3 fibers, cotton models are removed through the decomposition process (see Figure 5) [31] which begins below 300°C, and terminates close to 700°C.

The porous body at 100°C has a weight loss of 9% which indicates that only evaporation solvents are used in the TG analysis. A weight loss of 8% over a primary loss of 9% at 200°C indicates the evaporation of water in the sample and the beginning of the organic material evaporation. An additional weight loss of 55% at 400°C indicates the organic matter evaporation from the cotton fiber which forms the porous body [31]. A mass loss of 11% and a final weight of 83% in relation to the green body which indicates the total organic matter evaporation and the formation of the defined porous body.

Figure 4b shows morphological features of the porous ceramic of Al2O3-ZrO2, in which grain growth was possible due to the presence of ZrO2, causing the formation of diffusion barriers to control the growth and formation of the grain[32]. Furthermore, it was possible to verify the solid phase sintering, where the densification and grain growth are controlled by the diffusion across the grain boundary [33]. Therefore, it is reasonable to affirm that the formation of porosity in the structure is beneficial to decrease the thermal conductivity of the sample.

Table 1.

Physical and geometrical properties of the Al2O3 and Al2O3-ZrO2 porous ceramics.

For the porosity calculation by the average of 3 samples, the Al2O3 theoretical density value was assumed to be 3940 kg.m-3; the actual density was 2336 kg.m-3. A porosity average value of 40.7% and 77.9% was obtained for the Al2O3 and Al2O3-ZrO2 porous, respectively.

Table 2.

Analysis of thermal conductivity by laser flash method of the Al2O3 and Al2O3-ZrO2 porous bodies.

In the literature very few data about the thermal conductivity of the Al2O3 and ZrO2 system is available. However the effect of the porosity to reduce the thermal conductivity of ceramic materials is a well recognized phenomenon, that has been widely applied for Al2O3 and ZrO2 [3,10,34]. In addition to the absolute value of the porosity, the interconnection of grain size and pore shape have a significant influence on the final thermal conductivity. The high purity Al2O3 with no pores and with average grain size ~1 μm shows a thermal conductivity of approximately 33 W.m-1.K-1 at room temperature. High purity ZrO2 without porosity and average grain size of ~1 μm has a thermal conductivity of approximately 3.3 W.m-1.K-1 [35] at room temperature. Different studies show that thermal conductivity is independent of the grain size [36,37].

Some studies have been reported in the literature on the effect of porosity on reducing the thermal conductivity of solids, especially the porosity of Al2O3 [3,10]. In addition to its absolute value, the grain size and porous interconnectivity and shape have a significant influence on the final thermal conductivity. Porosity-free high-purity Al2O3 with a grain size ~1 μm has a thermal conductivity of approximately 33 W.m-1.K-1 at room temperature [35] which is very high when compared to the results obtained for porous Al2O3 reported by B. Nait-Ali et al [10] and Z. Zivcova et al [3]. Note that for the relative thermal conductivity the grain size dependence of thermal conductivity [36,37] is irrelevant, since it is cancelled out by taking the conductivity ratio [3].

Nait-Ali and co-workers [10] conducted a research on the Al2O3-ZrO2 system relating thermal conductivity to porosity. Their samples were sintered at 1400°C and pores were generated by a pore-forming polymer. Results showed that commercial Al2O3 with 40% of porosity presented a thermal conductivity of 9 W.m-1.K-1 and average grain size of 0.5 μm; for commercial ZrO2, thermal conductivity was 0.9 W.m-1.K-1 for 37% of porosity and average grain size of 0.1 μm.

The obtained Al2O3 porous bodies sintered at 1600°C have a thermal conductivity of 1.63 W.m-1.K-1 with a porosity of 40.71% and average grains size 0.55 μm, using cotton pore-forming agent and alumina obtained by a phase transition. Correlating these values and methods with literature data shows that Al2O3 porous bodies have high refractory properties from the combination of factors such as the synthesis method, grain size and porosity, when compared to the thermal conductivity of alumina bodies analyzed at temperatures as high as 1000°C [38].

Bansal and co-workers [39], using samples of commercial ZrO2-Al2O3 sintered at 1000°C with density around 99% with average grain size of ~1 μm showed a thermal conductivity of 6.9 W.m-1.K-1. The porous Al2O3-ZrO2 fibers obtained in this work were sintered at 1600°C and the calculated thermal conductivity was 1.61 K(W.m-1.K-1) with a porosity of 77.9% and an average grain size of ~1 μm. The cotton replicated fibers of Al2O3-ZrO2 sintered at 1600°C presented very low thermal conductivity compared to other works using different processes of pore formation.

4. Conclusions

Al2O3 porous bodies composed of ceramic fibers were successfully obtained by the embedded fibrous organic matrix method with cotton as a template. SEM at different temperatures during heat treatment along with thermogravimetric analysis data indicates a step-by-step method for the complete formation of the ceramic fiber porous body. The sintering temperature, low heating rate and the use of cotton as template had a strong effect on the surface area, pore size and distribution of the synthesized fibers. Thermal conductivity data show excellent results when compared to the literature, due to the direct influence of the organic template as a shape-model and the efficient method of synthesis. The results show that the Al2O3 and Al2O3-ZrO2 porous body are an excellent thermal insulator with direct application for refractories. A higher porosity and lower densification of the porous body is made possible with the addition of ZrO2 to the Al2O3 matrix. However, there was no difference in thermal conductivity due to the characteristic values of density, specific heat and microstructure observed in both materials.